Cracks are the most common type of problem in any type of building. However, most people don’t know whether they should be concerned. Due to faulty steps during construction or unavoidable reasons different types of cracks form in parts of the building over a length of time. In this blog post, we will look at some of the types of cracks that can occur, what they mean and what type of repair is necessary.
Active or Dormant?
Cracks can be defined into two main categories – Active or Dormant.
If cracks are active, they show some movement in width, depth or direction over a measured period of time. These are different from dormant cracks, which remain unchanged over a measured period of time. Active cracks tend to be more serious than dormant cracks, as some active cracks can be structurally hazardous. This makes timely identification and the adoption of preventative measures essential to the integrity of the building. However, if dormant cracks are left unrepaired they can lead to a variety of different problems such as moisture penetration.
So, what causes cracking?
So we know that cracks are active or dormant, but why did they form in the first place? Below, we will look at the most common causes of cracking in buildings.
Subsidence is simply the downward movement of the soil beneath a building. By far the most common cause of subsidence is the action of tree roots on shrinkable clay-rich soils. If a tree is allowed to grow in close proximity to a house which sits on clay, the tree will draw up considerable amounts of water which will cause the clay to shrink and the house to subside. Another common cause of subsidence are broken drains which cause water to leak out into the clay which over time weakens it significantly.
Subsidence cracks are most commonly wider at the top of a property. The cracks will generally travel diagonally in opposition to the downward movement.
To repair subsidence the tree must be removed in stages (to avoid heave – below) and the property underpinned. Significant cracks can then be repaired and repointed.
Heave is the opposite of subsidence, meaning that it is the upward movement of soil. This is caused as a result of dry soils expanding due to water being reintroduced. The most common situation when heave occurs is when a large tree is removed and the water which it would have drawn up is now introduced back into the soil. This is why trees should be removed in stages over a period of time to avoid sudden heave.
Where heave occurs cracking is normally wider at the bottom of the property. The cracks taper as it rises and cracks generally occur at right angles to the diagonal and upward movement.
Settlement is the downward movement of a property which has been newly constructed. This can also occur if you extend your property and cause additional load to be applied to the soils. This is a process of soil consolidation and tends to occur once over a period of time and then cease. This must be carefully controlled in the design of a new property, but generally should only be considered worrying if there is a major error in construction or design.
If cracking has been accommodated then decorative repairs only will normally be necessary. If the cracks are very significant then some more intense repairs may be involved (repointing, wall-ties, etc.)
4. Thermal Movement
All materials can expand and contract as they warm up and cool down. In modern construction, this is dealt with by movement joints so that cracking occurs exactly where you want it to rather than areas that would cause an issue to the integrity of the building. In older buildings, thermal movement can cause problems where large expanses of brickwork or flat roofs are subject to direct sunlight, as the construction methods used where different.
Thermal movement may be difficult to analyse due to the materials, angle of the material relative to the sun and its age. Various forms of repair can be required or methods put in place to minimise the impact of thermal changes or reduce temperature change altogether in the future.
5. Seasonal Movement
In the UK, we are particularly subject to the four seasons which will affect shrinkable soils (clay and silts) in different ways. In the summer clay will shrink as it dries out, and some soils will expand during the winter months with heavy rainfall. Older properties will have relatively shallow foundations and will be more subject to seasonal movement than newer properties. Seasonal movement causes cracks which open and close throughout the varying seasons. These cracks are generally insignificant and can be dealt with through standard decoration.